D50C Twin Bonanza
First Impressions: The Twin Bonanza is a large, luxurious first-generation modern business twin with handsome lines and an excellent safety record. It is remarkably easy to fly and is a stable, single-pilot IFR platform. While rather quiet in the cabin, those listening outside hear a rich, head-turning resonance produced by its geared 295 horsepower engines, low RPM props and augmenter tubes. The Beechcraft Model 50 has stood the test of time and remains a relevant (and efficient) medium-sized twin.
Design Features: The “Twin Bo” or “T-Bone” is rightly declared to be the “grandfather of the King Air” and the lineage is easy to see. The name “Twin Bonanza” is a little misleading and is not a Bonanza with two engines! If you’ve never seen a Model 50 in person, it’s bigger than you’d expect. It is significantly larger (in width, height and length) than the single-engine Bonanza family and can comfortably carry up to eight people. Although the front cockpit and cabin have similar profile lines to the single-engine Bonanza, as well as some identical flight controls, the similarities end there. Walter Beech and his design engineers wanted a rugged and safe plane that was stable and easy to fly. A unique design feature is the T-Bone’s strength. It is one of very few light twins that is certified in the utility category (4.4 G) and tested to an 8 G load limit!
Ergonomics: It is hard to beat the comfort of the Twin Bonanza. Early models featured trailing edge steps and a copilot side door but later models have an airstair door on the aft right fuselage that provides the convenience and feel of an airliner. There are a variety of seating configuration options, including a sideways-facing couch that seats three. Card tables, club seating and even a beverage and ice bucket station are other luxury options.
To enter the cockpit, walk down the 12 inch-wide isle, step over the leading edge spar and then sit in either the pilot or copilot seat. If you are the acting copilot, things can be a bit confusing when you sit on the right. On many Model 50s, the rudder pedals are in the middle (next to the pilot), not on the far right as generally expected. If the copilot needs to take the controls, he or she must reach down to unlock the seat, slide left to get cozy with the pilot and then pivot the throwover yoke to the right. Aside from this unique arrangement, the cabin is wide, the seats are comfortable, and the visibility is outstanding through the large double-pane glass! Heating is provided by a Janitrol heater, fueled from the right main tank and with ram air flow from a gap around the landing light housing on the tip of the nose cone.
Background: Beechcraft designed the Model 50 in 1949 and produced 975 aircraft between 1951-1963. Safety is carefully planned in the design of the Twin Bonanza. Interestingly, it was the first production aircraft with a shoulder belt. With its large nose, high landing gear and most of the aircraft mass forward and below the cabin, this plane is remarkably survivable in a crash. Case in point, when Beechcraft was demonstrating the Model 50 to the U.S. Army at an austere 1,700-foot strip near Fort Bragg, N.C., the factory demo pilot stalled out and crashed from a height of 50 feet. The Twin Bo was severely damaged but the pilot and five Soldiers walked away without injury. The Army was so impressed with the survivability, they ordered a steady stream of Twin Bonanzas, which was first designated the L-23 and later, the U-8 Seminole. The L-23/U-8 served the U.S. Army for almost forty years and made up the largest portion of their fixed-wing general utility inventory.
The Army used the L-23/U-8 Seminole for a variety of missions other than transportation. In Vietnam, the 138th Aviation Company used LR-23D/RU-8Ds equipped with large radio reception antennae mounted on the wingtips to triangulate and geolocate enemy forces. Their brave three-man crews (pilot, copilot and radio operator) spent many hours flying over Vietnam.
Powerplant: Most Twin Bonanzas have either the normally-aspirated 295 horsepower GO-480 or the supercharged 340 horsepower Lycoming GSO-480 or the GSIO-280. With the “G” meaning “Geared”, there is a gear-reduction box between the crankcase and the prop hub to reduce the prop RPM from the engine RPM by 36%. Another unique feature of the Model 50 its lack of cowl flaps. Augmenter tubes use exhaust gas tubes to produce a low pressure that draws cooling air through the cowling. Relatively low prop RPM, coupled with the augmenter tubes, create that beautiful Twin Bo music. The engines use a Bendix pressure carburetor which automatically compensates fuel flow to pressure altitude. This produces a perfect fuel to air ratio and the mixture controls are always kept full forward, further reducing the pilot’s workload. Two 12-volt batteries rigged in series provide 24-volt start, back-up and fill-in power for two 70-amp generators.
An interesting option available from the factory was “Junior JATO” bottles, manufactured by the Aerojet Company! The optional JATO bottles were attached at the rear of the engine nacelle on top of the wing and could be fired in the case of an engine failure in order to clear obstacles and propel the aircraft to a safe speed. This would be quite a kick in the pants but as far as I’m aware, this feature was never used other than in testing.
Preflight/Startup/Taxi: Engine preflight is simple with hinged engine cowls and mechanics love the spaciousness of the engine nacelles. A geared engine necessitates regular checks for gear wear. This is done by slightly manipulating a prop blade back and forth, checking for less than ½ inch of back-and-forth movement, measured at 48 inches from the hub. Weight and balance should be carefully calculated, taking into account the nose baggage compartment, seating configuration and fuel in each of the aircraft’s six bladder tanks.
After boarding the airstair, ensure it is completely latched with the safety chain attached at the top. Cockpit familiarity will help in finding hidden circuit breakers panels and the brain-teasing Beechcraft landing gear piano key switch. After priming, start with only the left magneto since it is the only mag with an impulse coupling.
Flight Characteristics: The Twin Bonanza is truly an easy and stable airplane to fly. Very little adverse yaw occurs during turns and almost no rudder is required. Single pilot IFR operation of many light twins can be a handful but the Twin Bo minimizes the workload and trims up nicely even without autopilot use. While not certified into “known icing” conditions, the props are equipped with a glycol “slinger ring” to prevent ice accumulation. 65% power in cruise (20 in manifold pressure with 2600 engine RPM) will produce about 160-165 knots in cruise at about 24 gallons per hour. While the speed isn’t eye-watering, keep in mind you’re flying a 6,300-pound aircraft with up to seven of your family and friends!
Operating a geared engine requires some additional understanding and training. It cannot be flown like a normal reciprocating engine. Always allow the engine to drive the prop, rather than the prop drive the engine. In other words, do not pull the throttles back to idle in flight, driving the manifold pressure back so far that the engine RPM decays with it. This creates gear “backlash” and causes premature (and costly) wear and tear on the gearbox. This technique also translates to the taxi. It is important to taxi with around 1300 RPM on the engines. When necessary to slowdown or stop, use the brakes. Brake pads are much cheaper than gearbox overhauls! A rule of thumb that Twin Bo pilots use while flying is to not let the airspeed exceed the manifold pressure (e.g. stay less than 170 MPH with 17” of manifold pressure)
The landing gear and flaps can be lowered at less than 150 mph and gear down indications are confirmed via three green lights and a nose gear “Down” flag on the front of the cockpit floor. Flaps are manipulated down to 30 degrees with a piano key switch and will decrease stall speed by 11 mph. If required, the landing gear can be manually extended with a mechanical ratcheting gear “pump” handle between the pilot and copilot seats, extending the landing gear via a jackscrew in about 150 actuations. The landing gear tires do not completely retract into the fuselage and, according to the Army, a pilot can actually land with the gear up and still maintain directional control with differential braking! A tailskid protects the tail so if the aircraft is equipped with original two-bladed props and the pilot shuts down the engines on short final and uses the starter to place them horizontally it’s theoretically possible to land a Twin Bonanza with the gear up and have no damage! However, this is not the author’s recommended technique!
Approach and landing are easy. The tall landing gear makes the plane sit high, so be prepared for a higher flare than in most light twins. It is important to note that the Pilot’s Operating Handbook states that Vmc is 89 mph, but the obstacle clearance airspeed is 80 mph for both short field takeoffs and landings. Should an engine fail while holding 80 mph, you will be in a very bad spot. Prudence says that you should only fly out of fields where you can climb out at Vmc or greater! With an engine secured and the prop feathered, a T-Bone should climb between 300-450 fpm at sea level, depending on gross weight.
Wrap-Up: The Twin Bonanza is a comfortable plane for both pilot and passengers and provides margins of safety rare in small twins. Affordability and efficiency are by-products of a well thought-out Beechcraft design. I’m confident Twin Bonanza owners will keep these airframes running for many years to come!